The present invention relates to a single cell of a flat plate type solid oxide fuel cell and a cell stack utilizing this single cell. More particularly, the present invention relates to improvement in the structure of a single cell.
A solid oxide fuel cell includes an air electrode to which air containing oxygen is supplied from the outside, a fuel electrode to which fuel gas such as hydrogen, carbon monoxide or methane is supplied from the outside, and an electrolyte through which oxygen ions can pass between these electrodes, and is constituted to generate electric power by electrochemical reaction of fuel gas and oxygen ions. At the air electrode, electrons supplied from an external circuit react with oxygen in air so that oxygen ions are produced. Then, the oxygen ions pass through the electrolyte to reach the fuel electrode. Further, at the fuel electrode, the oxygen ions having passed through the electrolyte react with fuel gas so that products are emitted and electrons are supplied to an external circuit.
As one conformation of this solid oxide fuel cell, there is conventionally a flat plate type. In the flat plate type solid oxide fuel cell, a fuel electrode film is formed on one surface (for example, a front surface) of an electrolyte plate, and an air electrode film is formed on the other surface (for example, a back surface), thereby forming a single cell. Then, a plurality of single cells are superposed each other with a separator plate having a gas flow path formed thereto being sandwiched therebetween, thereby forming a laminated body. Furthermore, when a metal manifold plate for distributing gas in accordance with each electrode is attached around the laminated body, a cell stack is formed.
In this flat plate type solid oxide fuel cell, however, since the structural strength of the cell stack is assured by the electrolyte plate and the separator plate, the extremely high accuracy of the dimension or flatness of the electrolyte plate or the separator plate is demanded in order to prevent the cell stack from being damaged due to the thermal stress generated during heating such as power generation. The processing cost of the electrolyte plate and the separator plate is therefore increased, and the quality control of these plates must be performed strictly.
Moreover, since the separator plate and the single cell are superposed as separate members, the electric resistance between these members becomes large, and large losses are generated in the electrical output from the stack.
In addition, since the separator plate has flow paths between respective electrodes by providing, e.g., ribs in order to distribute gas, the separator plate does not have enough strength and is apt to be damaged. Additionally, its processing cost is increased.
Further, since a material used for the separator plate is relatively expensive and must have an enough thickness in order to assure the strength, the material cost of the cell stack is increased.
Furthermore, since the manifold plate provided around the laminated body constituted by superposing the single cells is made of metal, the thermal expansion coefficient of the manifold plate is greatly different from that of the laminated body of the single cells. Therefore, the thermal stress is produced between the laminated body and the manifold plate during electric power generation, which can be a factor of damage to the cell and stack.
It is an object of the present invention to provide a single cell of a flat plate type solid oxide fuel cell and a cell stack utilizing this single cell which can improve the strength of the cell and/or stack and facilitate processing of components to reduce the processing cost. Moreover, it is another object of the present invention to provide a single cell of a flat plate type solid oxide fuel cell and a stack utilizing this single cell which can improve the output characteristic and reduce the material cost.
To achieve this aim, according to the present invention, there is provided a single cell of a flat plate type solid oxide fuel cell, the single cell comprising: a first electrode member consisting of a porous substrate; an electrolyte film formed on any one of a front surface and a back surface of the first electrode member; a second electrode member formed on the electrolyte film; and a separator film formed on the other surface of the first electrode member, wherein the first electrode member is one of a fuel electrode and an air electrode, and the second electrode member is the other one of the fuel electrode and the air electrode.
Therefore, since the first electrode member is formed by a porous substrate, gas can come into contact with the first electrode member while passing through the inside of the first electrode member, thereby causing a reaction. Thus, since ribs or the like for forming a gas flow paths do not have to be provided, the structure of the single cell can be simplified, and the strength can be improved. As a result, the size of the single cell can be increased, and the power generation characteristic can be hence enhanced. In addition, since the gas passes through the inside of the porous substrate, a contact area of the first electrode member with respect to air per unit capacity can be enlarged. Consequently, the power generation performance by the single cell can be improved.
Additionally, since the first electrode member is formed by the porous substrate, the first electrode member can be softer than an solid matter composed of a material forming the electrode member. Accordingly, since the thermal stress of the cell and stack obtained by superposing the single cells during the power generation operation can be absorbed and alleviated, the high flexibility of the cell stack can be obtained, thereby improving the strength. Further, since the high processing accuracy of the single cell for avoiding damage due to the thermal stress is no longer necessary, the production cost can be reduced, and the quality control can be facilitated.
Furthermore, since a separator film is directly formed to the first electrode member consisting of the porous substrate, the separator film enters multiple minute holes on the surface of the first electrode member, and contact areas between the first electrode member and the separator film can be thereby increased. As a result, the contact resistance (electrical resistance at contact parts) between the first electrode member and the separator can be greatly reduced, thereby improving the power generation performance.
In addition, since both the separator and the electrolyte are films, for example, thin films each having a thickness of a several ten micron, the power losses caused due to the internal resistance of these films can be reduced. As a result, the power generation performance can be improved. Additionally, since the separator is formed of a film, a quantity of an expensive separator material to be used can be decreased. Consequently, the manufacturing cost of the single cell can be reduced.
Further, according to the present invention, in the single cell of the flat plate type solid oxide fuel cell, at least one of a part of the electrolyte film and a part of the separator film is a seal portion which covers all or a part of side surfaces of the first electrode member and functions as a gas seal film.
Therefore, since the inside and the outside of the first electrode member are shielded by the electrolyte film and the separator film, and the seal portion, the air or the fuel gas can pass through the inside of the first electrode member without wastefully leaking it out of the first electrode member.
Furthermore, according to the present invention, in the single cell of the flat plate type solid oxide fuel cell, the seal portion includes: a side film portion which covers each entire area of one of two pairs of opposed side surfaces of the first electrode member and seals that pair of side surfaces to prevent the gas from escaping; and a corner film portion which covers both end portions of the other pair of opposed side surfaces of the first electrode portion close to the former pair of opposed side surfaces and seals a part between those both end portions except a gas inflow/outflow opening to prevent the gas from escaping.
Therefore, the one pair of side surfaces of the first electrode member are covered with the side film portions, and the both end portions of the other pair of side surfaces are covered with the corner film portions, and it is hence possible to prevent the flow gas from leaking from parts of the side surfaces of the first electrode member except the gas inflow/outflow opening.
Here, in the single cell of the flat plate type solid oxide fuel cell according to the present invention, preferably at least one of the electrolyte film and the separator film, or more preferably both of these films are formed to the first electrode member by a wet process. In this case, since the films can be formed by the wet process at the low cost, the inexpensive single cell can be obtained.
On the other hand, according to the present invention, there is provided a cell stack of a flat plate type solid oxide fuel cell, the stack comprising a laminated body which is constituted by superposing the above-described single cells in series and has a conductive spacer provided between adjacent single cells in the lamination direction.
Therefore, since the gas can pass through the spacer portion, the gas can be supplied to the second electrode member, thereby generating power.
Moreover, in the cell and stack of the flat plate type solid oxide fuel cell according to the present invention, the spacer is a porous substrate.
Therefore, since ribs or the like for forming gas flow paths do not have to be provided to the spacer, the structure of the single cell can be simplified, and the strength can be improved. In addition, since the spacer can be formed soft by the porous structure, the thermal stress between the superposed single cells during the power generation operation can be absorbed and alleviated so that the high flexibility of the cell stack can be obtained, thereby improving the strength. Additionally, since the high processing accuracy of the single cell for avoiding damage due to the thermal stress is no longer necessary, the manufacturing cost can be reduced.
Further, in the cell stack of the flat plate type solid oxide fuel cell according to the present invention, the porous substrate as the spacer is formed of a material which is the same as that of the second electrode member.
In such a case, it is possible to suppress generation of the thermal stress caused due to a difference in the coefficient of thermal expansion or the like between the spacer and the second electrode member. Furthermore, since the spacer can function as a part of the second electrode member, thereby improving the power generation performance.
Moreover, in the cell stack of the flat plate type solid oxide fuel cell according to the present invention, a conductive jointing material is provided between the spacer and the separator film opposed to each other in adjacent single cells.
Accordingly, since a contact area of the spacer and the separator film of the single cells which are adjacent to each other in the lamination direction can be enlarged, the contact resistance between these members can be reduced. As a result, the power generation performance by the stack can be improved.
In addition, in the cell and/or stack of the flat plate type solid oxide fuel cell according to the present invention, manifold plates composed of ceramics are attached to side surfaces of the laminated body.
The thermal expansion coefficient of the manifold plate can be, therefore, equal to that of the laminated body. As a result, generation of the thermal stress during power generation or the like can be suppressed, and the cell and stack can be prevented from being destroyed.
Here, in the cell and stack of the flat plate type solid oxide fuel cell according to the present invention, the ceramics forming the manifold late is free-cutting glass ceramics.
Therefore, since the manifold plate can be easily cut, the laminated body can be assembled, and the manifold plates can be provided around the laminated body. Thereafter, through holes for gas can be formed in the manifold plates. Here the hole forming operation can be performed by attaching the manifold plates to the laminated body in advance and confirming the positions at which the through holes for gas are to be formed while looking toward side surfaces of the laminated body on which no manifold plate is attached. Therefore, through holes do not have to be forced in the manifold plate in advance, and the accuracy of the positions of the through holes can be readily improved. Consequently, since the through holes for gas do not have to be previously formed to the manifold plate, before attaching it, based on measurement values of the laminated body as in the prior art, the high processing accuracy is no longer necessary. As a result, the manufacturing cost can be reduced, and the quality control can be facilitated.
Further, in the cell stack of the flat plate type solid oxide fuel cell according to the present invention, the lamination direction of the laminated body is horizontal, and the first electrode member and the spacer are arranged so as to be orthogonal to the lamination direction.
Here, in the cell stack according to the present invention, since the structural strength of the laminated body is secured by the first electrode member and the spacer, the higher strength can be obtained as compared with the case where the strength is assured by the electrolyte plate as in the prior art. Thus, even if the lamination direction of the laminated body is set to be horizontal, the sufficient strength with respect to warpage can be obtained. Furthermore, by setting the lamination direction of the laminated body to be horizontal like above, flow directions of the fuel gas and the air can be orthogonal or horizontal. Accordingly, a degree of freedom of installation of the cell stack can be improved.